CAMBRIDGE, Mass. - October 18, 2007 - Acceleron Pharma, Inc., a biopharmaceutical company developing novel therapeutics that modulate the growth of tissues including bone and muscle, today announced results of a preclinical study with ACE-031, the company's lead product for the treatment of muscle dystrophy, amyotrophic lateral sclerosis and cancer-related muscle loss, demonstrated that ACE-031 increased muscle mass despite concurrent treatment with glucocorticoids. These data were presented at the 12th International Congress of the World Muscle Society in Sicily, Italy.

In this preclinical mouse model, treatment with dexamethasone resulted in weight loss and a 5% reduction in lean body mass. Treatment with ACE-031 resulted in a 19% increase in lean body mass. Combination treatment with dexamethasone and ACE-031 resulted in an 11% increase in lean body mass, showing that ACE-031 was able to offset the muscle loss associated with glucocorticoid treatment.

"Previous studies with ACE-031 demonstrated that the drug directly enhances muscle mass and strength in preclinical models of neuromuscular diseases. These new data provide yet another example of the ability of ACE-031 to prevent muscle loss or restore lost muscle resulting from a disease or its treatment and further support the potential of ACE-031 to benefit patients suffering from neuromuscular disease or muscle loss associated with cancer", said Jasbir Seehra, Ph.D., Chief Scientific Officer at Acceleron. "We look forward to advancing ACE-031 into clinical studies in early 2008 and further exploring the potential of this drug as a treatment for patients suffering from muscle loss."

Glucocorticoids, such as prednisone or dexamethasone, are used to treat numerous diseases including Duchenne's muscular dystrophy and various cancers. The chronic use of glucocorticoid treatment causes significant side effects including muscular atrophy and loss of muscle strength.

About ACE-031

ACE-031, a protein therapeutic based on the activin receptor type IIB (ActRIIB), is the first of several compounds Acceleron is developing for the treatment of diseases involving the loss of muscle mass, strength and function. ActRIIB binds to myostatin (GDF-8) and other negative regulators of muscle mass and strength. Over-expression of myostatin has been shown to cause a loss in muscle mass and strength, and inhibition of myostatin results in the selective increase in skeletal muscle mass and strength. In preclinical models, ACE-031 increased skeletal muscle mass and strength in disease models of amyotrophic lateral sclerosis (ALS), muscular dystrophy, glucocorticoid-induced muscle loss and age-related muscle loss (sarcopenia). ACE-031 offers the potential for an entirely new type of therapy for the treatment of various muscle wasting conditions.

About Muscle Loss

Loss of muscle mass and strength is a serious clinical consequence of several types of diseases and conditions: (1) neuromuscular diseases, such as ALS and muscular dystrophy, in which patients suffer from the loss of muscle strength and function which ultimately leads to respiratory failure and death (2) cancer-related muscle loss, in which patients can lose muscle mass and body weight as a result of either their cancer treatment (hormone deprivation therapy causes loss of muscle) or the cancer itself (3) other cachexias, in which the muscle loss is associated with chronic diseases such as congestive heart failure, chronic obstructive pulmonary disease, and chronic kidney disease and, (4) age-related muscle loss, known as sarcopenia, in which generalized muscle loss and frailty leads to frequent falls and morbidity and mortality from broken bones. A drug candidate that would increase muscle mass and strength is expected to produce a major clinical benefit across this broad range of serious medical conditions in which there are currently no safe and effective therapies that selectively increase skeletal muscle mass and strength.

About Acceleron Pharma

Acceleron is a privately held biopharmaceutical company committed to discover, develop, manufacture and commercialize novel biotherapeutics that modulate the growth of bone, muscle, fat and the vasculature to treat musculoskeletal, metabolic and cancer-related diseases. Acceleron's scientific approach takes advantage of its unique insight into the regenerative powers of two protein families: the Growth and Differentiation Factors (GDFs) and Bone Morphogenetic Proteins (BMPs). ACE-011, a novel bone forming agent, is the company's lead program, and is being developed to reverse bone loss in diseases such as cancer-related bone loss. In addition, the company is advancing through preclinical development product candidates that increase muscle mass, control angiogenesis and inhibit fat accumulation. Acceleron utilizes proven biotherapeutic technologies and capitalizes on the company's internal GMP manufacturing capability to rapidly and efficiently advance its therapeutic programs. The investors in Acceleron are Advanced Technology Ventures, Flagship Ventures, OrbiMed Advisors, Polaris Ventures, Sutter Hill Ventures and Venrock.